This invention relates to a pedal-operated drive unit for bicycles or the like.
As is well known, a bicycle normally comprises a pair of pedal cranks (or levers) having a common rotation pivot of transverse axis, at their ends there being positioned two respective pedals rotating about axes parallel to the pivot axis. The position of the pedal axes as the angular position of the pedal cranks varies defines a perfect circumference, the radius of which is currently dimensioned, for an adult of average stature, at 170 mm, this being the best compromise between transmission of muscular force to the wheels and frequency of pedalling.
In practice, cyclists taking part in sporting activities maintain a rhythm of 80-100 r.p.m. on the flat, this ensuring optimum comfort and roundness of pedalling (constant speed); on hills the rhythm is reduced, with maximum metabolic efficiency being sought at close to 60 r.p.m. This involves considerable muscular fatigue and, to reduce stress, the cyclist rises at intervals from the saddle to reduce muscle fatigue, especially of the quadriceps and calf, by working with a greater angle at the knee.
To improve muscular performance and efficiency during pedalling, solutions have been proposed in which the pedal cranks are of variable length and work in combination with cam means able to vary said length on the basis of the angular position of the two pedal cranks. With varying angular position of the pedal crank the position of the pedal axes in the plane of vertical projection defines an endless closed curve different from a circumference. To improve performance through this curve, the pedal crank is elongated during the thrust stage, i.e. during that angular portion extending roughly from the top dead centre to the bottom dead centre, and is consequently shortened during the remaining portion extending roughly from the bottom dead centre to the top dead centre.
This lengthening of the lever during the forward thrust portion gives a sensation of greater downward thrust; however this increase in the lever length also produces an increase in the pedal speed during the downward thrust and a reduction in this speed. The resultant effect is a loss of roundness during pedalling, which is undesirable especially over long distances and during acceleration. Moreover, when the athlete produces sudden acceleration (spurts) by raising his body from the saddle, he is hindered by the reduced length of the rear part of the pedal stroke, which proportionally reduces the force transmitted during traction.
Generally, said solutions do not improve, but indeed worsen, the efficiency of the musculo-articular system, mainly because they reduce the angle between the leg and the thigh at the top dead centre (with consequent excessive stress on the musculo-articular system), and compel the cyclist to rise onto the pedals because his quadriceps are excessively stretched, and in order to reduce the lactic acid accumulation therein.
An object of this invention is to provide a drive unit using pedals of variable length, which is able to overcome said drawbacks and improve the working conditions of the musculo-articular system.
This and further objects are attained by the present invention as characterised in the claims.